1. Field of the Invention
The present invention relates generally to a method and assembly for sealing a draw furnace, and more particularly, to a sealing assembly and method for sealing the top of an optical waveguide draw furnace.
2. Technical Background
Relatively high temperature heat sources are required for drawing high strength, low loss optical waveguide fibers from high silica-content optical waveguide fiber preforms or blanks. Two of the predominant heat sources utilized for drawing such fibers are zirconia furnaces and graphite furnaces. Fiber draw furnaces generally operate at temperatures greater than about 1900xc2x0 C., typically as high as about 2050xc2x0 C.
A disadvantage associated with zirconia induction furnaces is that extended use and thermomechanical stresses due to poor thermal shock resistance cause cracks in the muffle and susceptor. This cracking causes zirconia particles to migrate from the inner surface of the furnace onto the preform and/or fiber being drawn from the preform resulting in substantially weakened fiber and unacceptable product losses. Moreover, zirconia induction furnaces are sensitive to rapid changes in temperature. Accordingly, significant amounts of time are necessary for increasing and decreasing the temperatures within the furnace. Rapid heating and cooling of the furnace results in fracturing of the zirconia muffle, which necessitates replacement of the muffle and results in significant furnace down time.
Such shortcomings lead to the development of graphite induction furnaces. Graphite induction furnaces typically include a graphite muffle which is insensitive to rapid changes in temperature and thus is less susceptible to cracking. It has been found, however, that graphite furnaces are susceptible to oxidation at temperatures approaching about 450xc2x0 C. and that oxidation is particularly problematic when the graphite is exposed to the high temperatures used to draw optical waveguide fibers. For this reason, among others, it is preferable to maintain an inert atmosphere within the draw furnace. Oxidation occurs when gasses from ambient atmosphere react with the solid carbon muffle at high temperatures according to the following reactions:
C+O2xe2x86x92CO2; andxe2x80x83xe2x80x83(1) 
C+CO2xe2x86x922CO.xe2x80x83xe2x80x83(2) 
A typical onset temperature for reaction (1) for a graphite grade used in a draw furnace is about 700xc2x0 C. Reaction (2) becomes significant above 900xc2x0 C. These reactions of the furnace muffle with oxygen and carbon dioxide cause the furnace muffle to be consumed, especially at elevated fiber drawing temperatures, and are referred to generally as CO events.
The graphite muffle material is a composite of graphite grains bonded together by a carbon binder matrix. It is believed that the binder material is more susceptible to oxidation than the graphite grains. Therefore, when the composite of the two materials is exposed to air, and thus oxygen at temperatures above the oxidation onset temperatures, the matrix binder material preferentially oxidizes. The graphite grains, having no binder left to hold them in place, are then free to fall away from the composite structure. It is believed that this mechanism causes graphite particulate to migrate from the muffle wall to the fiber preform and/or fiber during drawing.
Graphite particulate that becomes incorporated into the fiber during drawing causes unacceptable product losses due to point defects. Point defects manifest themselves as sharp attenuation increases in the signal transmitted through the fiber. Point defect product losses due to graphite particulate from draw furnaces can be greater than about 5%, which is an unacceptably high loss. Graphite particulate that has adhered to the fiber during the draw process also contributes to fiber breaks.
In order to reduce graphite particulation produced by oxidation of the graphite muffle material, and thus the number of resulting point defects, an inert gas is typically supplied to the interior of the furnace to prevent ambient air and other gases from entering the furnace. Unfortunately, there are a number of operations that occur during optical waveguide draw operations which present ambient air and other gases with the opportunity to enter the furnace despite delivery of high inert gas flow rates into the furnace. When a mistake is made during these draw operations, either by human error, by mechanical failure, or otherwise, ambient air and/or other gases are often permitted to enter the furnace and a CO event occurs.
Many of these CO events occur during movement or operation of the top hat assemblies presently used to close the opening in the top of a draw furnace during blank load, unload, and idle periods. Typical top hat assemblies include a viton/rulon seal that is positioned adjacent the top of the optical waveguide draw furnace when the top hat assembly is positioned over the downfeed handle and lowered onto the draw furnace. In addition, these top hat assemblies incorporate a plurality of hooks used in manually lowering and raising the top hat assembly. The hooks are specifically employed to raise the top hat assembly while the downfeed handle is being removed from the draw furnace. Currently, an operator must engage the hooks prior to removal of the downfeed handle from the draw furnace to ensure that the top hat assembly is being withdrawn from the top of the draw furnace as the downfeed handle and the remainder of the waveguide blank are removed from the hot zone of the draw furnace. Otherwise, that portion of the downfeed handle exiting the hot zone of the draw furnace will approach the top hat assembly, and the extreme heat retained by the downfeed handle will melt the viton/rulon or other sealing member of the top hat assembly. Often times, however, an operator fails to engage the hooks, which ultimately results in the melting and thus the premature replacement of the top hat sealing member.
Moreover, if an operator fails to disengage the hooks after the top hat assembly has been positioned on the top of the draw furnace, certain aspects of the fiber drawing operation itself can create CO events. Generally, once a seal is formed between the top hat and the top of the draw furnace, the bottom door assembly of the draw furnace is opened and fiber drawing commences. During the start of optical waveguide fiber draw, the optical fiber blank is typically partially raised back out of the hot zone of the draw furnace to stretch and thin the fiber in order to obtain a desired fiber diameter. If the hooks are still engaged with the top hat assembly during this time, the top hat raises off of the top of the draw furnace and the seal is broken. Since the bottom door assembly of the draw furnace is open during fiber draw, the broken top hat seal creates what is known in the art as a chimney effect. The chimney effect is the rapid intake of air through the bottom opening in the draw furnace caused by, inter alia, the opening of the top of the draw furnace. Ambient air is thus forced into the muffle of the draw furnace and the oxygen contained within the air reacts with the carbon muffle causing a CO event. The graphite particulate is then free to migrate to the fiber which results in fiber being formed with an unacceptable number of point defects. In addition, the entrance of air into the draw furnace during fiber draw also results in diameter upsets which also creates unusable fiber. Moreover, each CO event degrades the draw furnace muffle which in turn reduces draw furnace efficiency and necessitates muffle replacement.
In view of these and other shortcomings, an improved assembly and method of sealing an optical waveguide draw furnace is needed to prevent the entrance of ambient air and other unwanted gases into the draw furnace.
One aspect of the present invention relates to an apparatus for cooperating with a downfeed handle to seal the top of an optical waveguide draw furnace. The apparatus includes an assembly that is constructed and arranged to removably cover the top of the draw furnace and mate with the downfeed handle. The assembly includes an elongated sleeve having a base and defining a chamber for receiving the downfeed handle. The apparatus further includes an inert gas supply which communicates with the assembly to selectively deliver inert gas into the chamber.
In another aspect, the invention relates to an apparatus for cooperating with a downfeed handle to seal the top of an optical waveguide draw furnace. The apparatus includes an elongated sleeve having a top and a base, and defining a chamber for receiving the downfeed handle. The apparatus further includes a sealing mechanism positioned on the sleeve at a location remote from the base. The sealing mechanism is arranged with respect to the sleeve to mate with the downfeed handle received in the chamber.
In yet another aspect the present invention is directed to a method of sealing the top of an optical waveguide draw furnace. The method includes the steps of sliding an assembly onto a downfeed handle such that the assembly is supported on the downfeed handle. The assembly defines a chamber for receiving the downfeed handle, and has a base and a sealing mechanism remote from the base. The sealing mechanism cooperates with the downfeed handle to close one end of the chamber. The chamber is purged with an inert gas, and the downfeed handle is lowered into the optical waveguide draw furnace to position the assembly on the top of the draw furnace.
The top hat assembly of the present invention results in a number of advantages over other top hat assemblies presently available. Principally, the top hat assembly of the present invention significantly reduces the number of CO events resulting from air entering the top of the draw furnace, and/or air entering through the bottom of the draw furnace due to unintentional removal of the top hat assembly when the bottom door assembly is in an open position, particularly during fiber draw. The reduction of CO events significantly reduces the number and frequency of point defect losses in optical waveguide fibers drawn in furnaces, particularly those having graphite muffles, and further, prolongs the life of the graphite muffles employed in draw furnaces. In addition, because the top hat assembly of the present invention obviates the use of hooks, operator error is substantially eliminated during fiber draw operations. Moreover, the construction and arrangement of the sealing mechanism of the present invention significantly extends the seal life, which in turn reduces draw tower down time associated with the replacement of spent seals.
Additional advantages are provided by the sealing assembly and method of the present invention. The inert gas purge cooperating with the top hat assembly of the present invention displaces air trapped within the top hat assembly, which in turn prevents air from entering the top of the draw furnace during optical waveguide blank loading. Without the purge, air otherwise trapped in the top hat assembly would be drawn into the draw furnace when the bottom door assembly is opened to begin fiber draw. That air, once inside the furnace, causes diameter upsets and CO events. Thus, this embodiment of the present invention further reduces the number of diameter upsets and CO events during drawing operations.
A single CO event in a graphite draw furnace during fiber draw typically results in a relatively large percentage of the optical fiber having unacceptable levels of point defects. Accordingly, cleaning of the draw furnace is essential following a CO event. Generally, such cleaning operations result in the draw furnace being down for approximately 6 hours to facilitate such cleaning and servicing operations. A further advantage of the present invention is the reduction in the amount of draw tower down time due to the substantial reduction in the frequency of CO events. Less down time results in increased production of optical fiber, and particularly optical fiber having an insignificant number of point defects.
The top hat assembly and method of sealing the top on an optical waveguide draw furnace of the present invention significantly reduces both mechanical and human error during draw operations. Moreover, the assembly and method of the present invention eliminates the use of hooks with the top hat assembly, and thus minimizes operator interaction with the top hat assembly during drawing operations. Such an assembly and method will reduce the frequency with which sealing members in the assembly are currently replaced, reduce the number of diameter upsets, and will significantly reduce the number of CO events during drawing operations, particularly those resulting from the chimney effect.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.